When a water tank is $30\%$ full, it contains 27 gallons less than when it is $20\%$ empty. How many gallons of water does the tank hold when it is full?
$20\%$ empty is the same thing as $80\%$ full.  Thus, 27 gallons represents the difference between $80\%$ and $30\%$ of the tank, meaning that 27 gallons is $50\%$ of the tank.  Since half the tank is 27 gallons, the entire tank can hold $\boxed{54\text{ gallons}}$.